Liquid crystal display apparatuses having liquid crystal displays, for example, are widely used as apparatuses for displaying a color video image.
Driving methods for liquid crystal display panels encompass field sequential driving. The field sequential driving refers to a driving method for controlling a liquid crystal display panel so that three subframes which respectively correspond to three primary colors are sequentially displayed in sync with respective lighting timings of backlights which respectively correspond to the three primary colors. The three subframes displayed in a time-division manner are superimposed on a retina of a viewer due to persistence of vision so that the three subframes are recognized by the viewer as one color frame.
FIG. 7 is a block diagram (cited from Patent Literature 1) illustrating an arrangement of a conventional drive circuit for driving a liquid crystal display panel by field sequential driving. The conventional drive circuit has an RGB sync separation circuit 42, an R scan speed conversion circuit 43, a G scan speed conversion circuit 44, a B scan speed conversion circuit 45, a timing control circuit 46, and a backlight drive circuit 47.
An input video signal 41 supplied to the conventional drive circuit is separated by the RGB sync separation circuit 42 into video signals which respectively correspond to the three primary colors, red (R), green (G), and blue (B). The video signals are supplied to the R scan speed conversion circuit 43, the G scan speed conversion circuit 44, and the B scan speed conversion circuit 45, respectively, so that respective scan speeds of the video signals are converted into tripled ones. Then, the video signals subjected to this speed conversion are sequentially supplied to a light valve (liquid crystal display panel) in accordance with signals from the timing control circuit 46. The backlight drive circuit 47 supplies backlight control signals to backlights in accordance with the signals from the timing control circuit 46.
In a liquid crystal display apparatus which does not have color filters but carries out the field sequential driving, each of pixels corresponds to one picture element. That is, the liquid crystal display apparatus which drives its liquid crystal display by the field sequential driving makes it possible to obtain a resolution three times a resolution of a common liquid crystal display apparatus in which each of pixels corresponds to three picture elements. Further, the liquid crystal display apparatus makes it possible to obtain a transmittance three times a transmittance of the common liquid crystal display apparatus. This makes it possible to reduce power consumption for image display operation at a luminance to one-third of that of the common liquid crystal display apparatus to perform the image display operation at the same luminance.
On the other hand, there has been known that the liquid crystal display apparatus utilizing the field sequential driving causes a phenomenon called color breaking. The color breaking is such a phenomenon that in a case where a line of sight of a viewer follows an object moving on a display screen, sequentially-displayed subframes which respectively correspond to the three primary colors are not evenly superimposed on a retina of the viewer so that the viewer recognizes the three primary colors such that a color component corresponding to one of the subframes is emphasized.
The following describes the color breaking, with reference to drawings.
FIG. 2 is a view illustrating how a white object having a uniform luminance moves rightward along a horizontal line in a video image to be displayed on a liquid crystal display.
(a) of FIG. 8 is a schematic view illustrating video signals which are outputted from the RGB sync separation circuit 42 in a case where a video signal indicative of the video image illustrated in FIG. 2 is supplied to the RGB sync separation circuit 42. In (a) of FIG. 8, signs IR, IG, and IB indicate video signals which respectively correspond to red, green, and blue, and the signs have subscripts (n−1, n, and n+1) indicating corresponding frame numbers. A height of each of the video signals IR, IG, and IB indicates a luminance of an image. In this example, the white object has a uniform luminance. Accordingly, the video signals IR, IG, and IB have rectangular shapes having an equal height.
In (a) of FIG. 8, a vertical axis represents a temporal axis, and time progresses downward along the temporal axis. (a) of FIG. 8 shows, perpendicularly to the temporal axis, a rightward coordinate axis corresponding to the horizontal line of FIG. 2. (a) of FIG. 8 shows the video signals IR, IG, and IB along a direction perpendicular to both the temporal axis and the rightward coordinate axis. Note that arrangement of the synchronization input signals along the rightward coordinate axis does not indicate an actual arrangement of the pixels on the display screen illustrated in FIG. 2.
(b) of FIG. 8 is a schematic view illustrating output signals which are supplied from the R scan speed conversion circuit 43, the G scan speed conversion, circuit 44, and the B scan speed conversion circuit 45 to the light valve (liquid crystal display panel) in a case where the video signals IR, IG, and IB in (a) of FIG. 8 are supplied to the R scan speed conversion circuit 43, the G scan speed conversion circuit 44, and the B scan speed conversion circuit 45 on a frame-by-frame basis, respectively. The output signals are referred to as SR, SG, and SB, respectively, and have subscripts (n−1, n, and n+1) indicating that the output signals are those for different frames.
Consider here that a line of sight of a viewer follows an edge P of the white object in FIG. 2. The white object moves rightward along the horizontal line. Accordingly, a point of sight moves on the display screen while following the edge P. This movement corresponds to a downward movement of the point of sight along a dashed line (hereinafter, referred to as follow line Q) in (b) of FIG. 8.
In (b) of FIG. 8, the follow line Q intersects with end points of falls of the output signals SR. On the other hand, the follow line Q does not intersect with end points of falls of the output signals SG and SB. This indicates that the viewer recognizes only a red color at the edge P, and a green color and a blue color are off the point of sight.
Accordingly, the three primary colors cannot be properly superimposed on the retina of the viewer. As a result, the three primary colors are recognized at the edge P such that only red is emphasized. This is a phenomenon called color breaking.
The explanation above deals with a case where the line of sight of the viewer follows the edge of the white object moving on the display screen. However, the color breaking can be caused in case of objects other than the white object, and can also be caused in a position other than an edge of an object.
Patent Literature 1 discloses a liquid crystal display apparatus of a field sequential driving method. The liquid crystal display apparatus divides one frame into two subframes so as to display, in one of the two subframes, a subframe made up of a green component only, and display, in the other of the two subframes, a subframe made up of a red component and a blue component.
In order to realize such a display method, a liquid crystal display panel of the liquid crystal display apparatus has color filters which allow the red component and the green component to pass through, and color filters which allow the blue component and the green component to pass through. According to the liquid crystal display apparatus, it is sufficient to divide each frame into two subframes in total which are a subframe corresponding to green and a subframe corresponding to red and blue, in order to display an image in color. This makes it possible to increase a frame rate of a video image to be displayed on the liquid crystal display, as compared to that liquid crystal display apparatus of a field sequential driving method which requires three subframes. This makes it possible to expect an effect of alleviating the color breaking.
However, a problem still persists in that it is difficult to fundamentally alleviate the color breaking only by increasing a frame rate.
Patent Literature 2 discloses an image processing apparatus having a display position correction circuit which corrects display positions of subframes of each frame by use of a movement detector circuit. The display position correction circuit of the image processing apparatus corrects display positions of subframes so that subframes are evenly superimposed on a retina of a viewer while a line of sight of the viewer follows an object moving a display screen. This makes it possible to suppress color breaking in displaying a moving image.